The photos are downloaded as JPEGs in a Zip folder. Zip is a popular format widely used on the Internet. Zip files store one or several files in a compressed form that is easy to transfer and/or download.
Apple computers can open zip files easily just by double-clicking. Most other computers running windows are able to open and make the files available by right-clicking on the downloaded zip file and selecting either "unzip", "extract all", "decompress", or "uncompress".